The invention relates generally to nondestructive inspection techniques, and more particularly, to a thermographic nondestructive testing technique for determining flaws in an object.
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine cross-sectional thickness of cast metal and other solid objects. Conventionally, the object is probed with ultrasonic waves, which penetrate the surface and are reflected internally at the opposite side or surface of the object. Based upon the time required to receive a reflected wave, the distance to the opposite (back) side can be determined, giving the thickness of the object at that point. Unfortunately, conducting ultrasonic measurements of this sort to examine the cross-sectional thickness would usually necessitate a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface, a stream of liquid couplant must be applied to the surface or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are most often not very practical or even feasible for numerous structural and material reasons. For example, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can require substantial time delays, often of several hours.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Because heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. In contrast to most ultrasonic techniques, a transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one hundred times faster than most conventional ultrasonic methods if testing objects of large surface area.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame, which corresponds to a rectangular area, called a resolution element, on the surface of the object being imaged. Because the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast.
One application of transient thermography is for non-contact quantification of porosity, voids, and delaminations in thin-walled carbon fiber reinforced polymer composite aircraft structures. Determining porosity using thermography is based on the calculation of thermal diffusivity, which in turn requires thickness information. In all known contemporary techniques, a calibrated reference standard for thickness is required or temperature dependent images are required to be generated, which may intrinsically have greater error than required for accurate analysis.
Further, to determine all diffusivity components in one measurement, such as in case of anisotropic materials, various heat flow methods have been proposed that require full parametric fitting of spatio-temporal temperature data. The measured values of spatial temperature profiles can become unreliable when surface emissivity is not uniform or when surface reflections from nearby hot or cold objects are in the IR camera field-of-view.
A quantitative time of flight (tof) infrared thermography technique based on determination of the “inflection point” has been disclosed in commonly assigned U.S. patent Ser. No. 11/639,724, Ringermacher et al., “Method and apparatus for thermographic nondestructive evaluation of an object,” that obviates some of the aforementioned issues and provides diffusivity and thickness values.
However, there is a need for methods to determine diffusivity and porosity for composite articles that do not require exact thickness information, are not affected by surface emissivity variations or reflections, and do not require any sort of curve fitting to time-temperature data.